1. Field of the Invention
The present invention relates to a substrate for a light-emitting diode. More particularly, the present invention relates to a substrate for a light-emitting diode which can show a high heat dissipation capacity by achieving a low thermal resistance as the total thermal resistance of the whole substrate.
2. Description of Related Art
While a light-emitting diode (LED) is increasingly widely used recent years from viewpoints of energy saving etc., since an energy efficiency will fall when the temperature of an LED rises, it is important for an LED to efficiently release heat which is generated in association with light emission, and to prevent a temperature-rise of the LED. Then, in the art, various technologies for raising a heat dissipation capacity in a package and wiring substrate on which an LED is mounted have been proposed.
As an example of such technologies, for example, a technology for reducing a total thermal resistance of a whole substrate, by using as a wiring substrate a structure (heat slag type) in which an insulation layer is formed on a top of a metal base comprising (for instance, metals, such as aluminum, copper, silver and tungsten, and alloys of any of these metals, etc.) and a conductive pattern for an electrical connection with an LED is formed on a top of the insulation layer, or by using as a wiring substrate a structure (heat spreader type) in which a metal base is embedded in an insulation material with a conductive pattern for an electrical connection with an LED formed on its top, can be exemplified (for instance, refer to PTLs 1 and 2).
Since a substrate which comprises a metal base with a high thermal conductivity mentioned above has a lower thermal resistance as the total thermal resistance of the whole substrate as compared with a substrate which does not comprise such a metal base, it can more efficiently release heat which is generated in association with light emission of LED. However, an insulation layer formed on a top of a metal base generally comprises as a main material, dielectric materials, such as a resin and a ceramic, for example, and these dielectric materials have a lower thermal conductivity as compared with a metal base. A relatively low thermal conductivity, which such as insulation layer has, becomes a bottleneck in heat conduction between a conductive pattern and a metal base. Namely, when a heat dissipation capacity of a substrate is going to be further improved by further reducing a total thermal resistance of the whole substrate, there is a possibility that a relatively low thermal conductivity that an insulation layer has may become an obstacle.
The obstacle as mentioned above in a further improvement in a heat dissipation capacity of a substrate due to a relatively low thermal conductivity which an insulation layer has can be suppressed by reducing the thickness of the insulation layer, for example (for instance, refer to PTL 2). However, in a substrate according to a conventional technology, when a total thermal resistance of a whole substrate is going to be reduced by reducing a thickness of an insulation layer, there is a possibility of causing reduction of insulation reliability in the substrate. Specifically, in association with reduction of the thickness of an insulation layer, there is a possibility that it may become difficult to secure the electric insulation with a conductive pattern and a metal base and an electrical short (short-circuit) between conductive patterns through a metal base may be caused, for example.
On the other hand, although it is desirable that the thickness of an insulation layer is large from a viewpoint of securing insulation reliability of a substrate, the more the thickness of the insulation layer increases, the more the total thermal resistance of the whole substrate increases and the more the total heat dissipation capacity of the whole substrate falls due to the relatively low thermal conductivity which the insulation layer has. In addition, a way of the dimensional change accompanying a temperature change (hereinafter, may be referred to as a “thermal expansion shrinkage behavior”) is different between a dielectric material which constitutes an insulation layer (for instance, a resin, a ceramic, etc.) and a material which constitutes a metal base and a conductive pattern (for instance, a metal, etc.). Therefore, there is a possibility that a fissure (crack) may occur in an insulation layer due to the above-mentioned difference in thermal expansion shrinkage behavior and so on to cause a problem, such as reduction of reliability of a substrate under a high-humidity environment (high-humidity reliability), for instance, in connection with the temperature change of the substrate in a manufacturing process of a substrate, in an implementation process of a package which contains an LED, and in an operation period after completion of the package containing an LED, etc., for example. Moreover, the larger the thickness of a metal base is, the more such a concern becomes noticeable. Furthermore, when a ceramic is adopted as a dielectric material which constitutes an insulation layer and a substrate is manufactured by co-firing such an insulation layer with a metal base and a conductive pattern, such a concern becomes more noticeable.
As mentioned above, in the art, there is a demand for a substrate for a light-emitting diode which can show a high heat dissipation capacity by achieving a low thermal resistance as the total thermal resistance of the whole substrate, without reducing insulation reliability and high-humidity reliability of the substrate.